That error is because for some reason your MySQL statement failed.
Try this:
$query = "SELECT * FROM USERS WHERE UNICK = '".mysql_real_escape_string($uname)."'";
$result = mysql_query($query) or die('MySQL Error: '.mysql_error());
if(mysql_num_rows($result){
$msg=$msg."User Name already exists. Please try another one<BR>";
$status= "NOTOK";}
$query = "SELECT * FROM USERS WHERE UEMAIL = '".mysql_real_escape_string($email)."'";
$result = mysql_query($query) or die('MySQL Error: '.mysql_error());
if(mysql_num_rows($result){
$msg=$msg."Email is already registered. Only one account per email is allowed.<BR>";
$status= "NOTOK";}
You must trap MySQL Errors, the code given above may not be best result for production websites since it kills the website & spits out a MySQL Error.
There are lots of posts here on proper error trapping techniques, if you care to read up on it.
Also - you should NEVER trust user input - look into mysql_real_escape_string().
You should also look into Cross Site Scripting, especially if you are going to post back any user input.
